This invention relates generally to the operation of a cryogenic plant such as a cryogenic air separation plant.
Cryogenic plants such as cryogenic rectification plants frequently experience problems such as the operating power is too high or not optimal and there is no way to indicate in real-time/online that a problem has occurred and caused the plant to operate in a non-optimal fashion or to determine a solution in real-time/online. Moreover, scheduled shutdowns may occur more frequently than desired and little can be done to identify inefficiencies in time to avoid unscheduled shutdown. Furthermore, non-optimal product flows, recoveries or purity caused by instrumentation or primary control or advanced control failure or anomaly may occur and there is no way of determining whether product flows, recoveries or targeted purities are actually optimal or not, and it is difficult to determine the cause of deviation.
Accordingly, it is an object of this invention to provide a method for operating a cryogenic plant, such as a cryogenic air separation plant which will enable online real time monitoring of the plant and provide online real time diagnosis of problems along with suggested corrective actions to quickly address and correct such problem.
The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention which is:
A method for operating a cryogenic plant comprising:
(A) determining the expected optimal value range at a given time using a process model for at least one key performance indicator from the group consisting of plant power consumption, product recovery, PID loop performance, MPC steady state optimization, MPC model prediction and MPC dynamic performance;
(B) monitoring the value of each of the key performance indicators for which an expected optimal value range has been determined;
(C) alerting a plant operator when the value of a monitored key performance indicator is outside of the determined expected optimal value range for that key performance indicator; and
(D) suggesting at least one action to the plant operator for adjusting the operation of the cryogenic plant so that the value of the monitored key performance indicator which is outside of the expected optimal value range for that key performance indicator is changed so that it is within the expected optimal value range for that key performance indicator.
As used herein the term xe2x80x9ccolumnxe2x80x9d means a distillation or fractionation column or zone, i.e. a contacting column or zone, wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, as for example, by contacting of the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column and/or on packing elements such as structured or random packing. For a further discussion of distillation columns, see the Chemical Engineer""s Handbook, fifth edition, edited by R. H. Perry and C. H. Chilton, McGraw-Hill Book Company, New York, Section 13, The Continuous Distillation Process. 
The term xe2x80x9cdouble columnxe2x80x9d is used to mean a higher pressure column having its upper portion in heat exchange relation with the lower portion of a lower pressure column. A further discussion of double columns appears in Ruheman xe2x80x9cThe Separation of Gasesxe2x80x9d, Oxford University Press, 1949, Chapter VII, Commercial Air Separation.
Vapor and liquid contacting separation processes depend on the difference in vapor pressures for the components. The high vapor pressure (or more volatile or low boiling) component will tend to concentrate in the vapor phase whereas the low vapor pressure (or less volatile or high boiling) component will tend to concentrate in the liquid phase. Distillation is the separation process whereby heating of a liquid mixture can be used to concentrate the more volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Partial condensation is the separation process whereby cooling of a vapor mixture can be used to concentrate the more volatile components) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Rectification, or continuous distillation, is the separation process that combines successive partial vaporizations and condensations as obtained by a countercurrent treatment of the vapor and liquid phases. The countercurrent contacting of the vapor and liquid phases can be adiabatic or nonadiabatic and can include integral (stagewise) or differential (continuous) contact between the phases. Separation process arrangements that utilize the principles of rectification to separate mixtures are often interchangeably termed rectification columns, distillation columns, or fractionation columns. Cryogenic rectification is a rectification process carried out at least in part at temperatures at or below 150 degrees Kelvin (K).
As used herein the term xe2x80x9cindirect heat exchangexe2x80x9d means the bringing of two fluid streams into heat exchange relation without any physical contact or intermixing of the fluids with each other.
As used herein the term xe2x80x9cexpansionxe2x80x9d means to effect a reduction in pressure.
As used herein the term xe2x80x9cproduct gaseous oxygenxe2x80x9d means a gas having an oxygen concentration of at least 90 mole percent.
As used herein the term xe2x80x9cfeed airxe2x80x9d means a mixture comprising primarily oxygen, nitrogen and argon, such as ambient air.
As used herein the terms xe2x80x9cupper portionxe2x80x9d and xe2x80x9clower portionxe2x80x9d mean those sections of a column respectively above and below the mid point of the column.
As used herein the term xe2x80x9cMPCxe2x80x9d means model productive controller.
As used herein the term xe2x80x9cPIDxe2x80x9d means proportional integral derivative controller.
As used herein the term xe2x80x9ckey performance indicatorxe2x80x9d means a calculated parameter used to determine the performance of a cryogenic plant or a model predictive controller.
As used herein the term xe2x80x9csteady state optimizationxe2x80x9d means the ability of a model predictive controller to control a cryogenic plant and keep the controlled variables within a given range at steady state.
As used herein the term xe2x80x9cload changexe2x80x9d means the ability of a model predictive controller to control a cryogenic plant and keep the controlled variables within a given range when a load change is made.
As used herein the term xe2x80x9cplant operatorxe2x80x9d means personnel responsible to monitor and operate a cryogenic plant.
As used herein the term xe2x80x9cscreen displayxe2x80x9d means a computer screen acting as a human-machine interface residing on a plant control system computer to display real-time on-line data.
As used herein the term xe2x80x9ctop-down diagnostic tree methodologyxe2x80x9d means a method used to determine plant and MPC performance problems by starting at a generalized composite key performance indicator, breaking it down into sub-key performance indicators, and arriving at a particular branch as the root cause of the problem.